The invention relates to a pneumatic brake booster, in particular for motor vehicles.
Pneumatic brake boosters of said type are known, e.g. from DE 44 08 993 A1 and U.S. Pat. No. 5,732,556, which are incorporated by reference herein. Millions of them are fitted in motor vehicles and used to boost the pedal force, which is introduced into a vehicle brake system by a driver, and transmit it to a master cylinder in order in the latter to pressurize a hydraulic fluid. The hydraulic pressure thus generated is supplied to the vehicle brakes, where it presses friction elements against a rotating component which is to be braked, as a rule against a brake drum or brake disk.
A conventional vacuum brake booster utilizes the difference between atmospheric pressure and a vacuum generated artificiallyxe2x80x94e.g. by the intake system of an i.c. enginexe2x80x94for force, boosting. In the brake booster two chambers are provided for said purpose, of which the one is constantly connected to a vacuum source, while the other is connectable by means of a control valve either to atmospheric pressure or to a vacuum. The two chambers, which are mostly referred to as the working chamber and the vacuum chamber, are separated in a gasproof manner from one another by a movable wall. The movable wall is normally formed by a diaphragm of elastomeric material (e.g. rubber), which is supported by a plate-shaped component made of rigid material. For fastening the movable wall on the housing of the control valve, according to the already cited DE 44 08 993 A1 a cylindrical collar of the plate supporting the rubber diaphragm is pushed onto the control valve housing and then deformed in sections by radial caulking into an annular groove, which is provided in the control valve housing in the region overlapped by the collar. In said manner a keyed connection of the movable wall to the control valve housing is obtained. In force transmitting direction the supporting plate lies against a radial shoulder of the control valve housing, so that the force generated at the movable wall by the said pressure difference may be transmitted to the control valve housing and from the latter to a master cylinder disposed downstream of the brake booster.
Manufacture of the said annular groove of the control valve housing, into which the collar of the plate supporting the rubber diaphragm is at least in sections radially caulked, is relatively complex from the point of view of the moulds. Particularly when the control valve housing is manufactured by injection moulding, it is necessaryxe2x80x94because of the undercuts arising as a result of the annular groovexe2x80x94for the injection mould to contain moulding elements which are displaceable in transverse direction, i.e. at right angles to the longitudinal direction of the control valve housing, so that after injection moulding the control valve housing may be removed from the mould.
The object of the invention is to indicate a pneumatic brake booster, the control valve housing of which is less costly to produce without having to dispense with the fastening of the movable wall to the control valve housing achieved by simple caulking of the diaphragm plate.
Proceeding from the initially cited prior art, said object is achieved according to the invention by providing in the region of the control valve housing overlapped by the collar of the plate a plurality of recesses, which are spaced apart from one another in peripheral direction and into which the collar of the plate is radially deformed, e.g. by caulking, wherein the recesses are accessible from the end of the control valve housing disposed in the booster housing. In other words, the control valve housing of the brake booster according to the invention has recesses, which extend from the end face of the control valve housing situated in the booster housing as far as into the region, which is overlapped by the collar of the plate. Radially, said recesses are positioned in such a way that the wall of the control valve housing in the region overlapped by the collar is interrupted by the recesses, i.e. in the portion of the control valve housing overlapped by the collar each recess gives rise to an opening, also referred to as a caulking window, into which the collar may be radially deformed. Preferably, the recesses extend in the control valve housing in axial direction parallel to the latter""s longitudinal axis. Alternatively, the recesses may extend obliquely relative to the longitudinal axis of the control valve housing, it merely being necessary to ensure that each recess creates an opening in the region of the control valve housing overlapped by the collar.
Since according to the invention the said recesses in the control valve housing may be manufactured from one end of the latter, there is no need for the transversely displaceable elements required in prior art so that, now, a plurality of so-called mould cavities, i.e. a plurality of injection moulds for control valve housings, may be disposed alongside one another in a single moulding tool. Previously, this was not possible within an acceptable amount of space because the transversely displaceable elements took up too much room. According to the invention, therefore, with only one moulding tool and a correspondingly low spatial requirement a plurality of control valve housings may be manufactured simultaneously, thereby significantly reducing production costs.
According to a preferred development of the brake booster according to the invention, in the recesses a radially outer region of the latter at the end of the control valve housing disposed in the booster housing forms undercuts for spring arms of a retaining element, which is used for positioning a force output tappet of the brake booster. The said undercuts are accessible from the direction of the other end of the control valve housing, i.e. the end disposed outside of the booster housing, particularly in a direction parallel to the longitudinal axis of the control valve housing. This means that the undercuts may be manufactured from the other end of the control valve housing, with the result that their manufacture likewise does not require transversely displaceable elements in the manufacturing tool.
The already mentioned retaining element, which is to position the force output tappet of the brake booster on the control valve housing, comprises spring arms which are preferably designed in such a way that the retaining element is introducible into the control valve housing from the end of the latter disposed in the booster housing, wherein the spring arms then latch behind the undercuts formed by the recesses. The spring arms, as they are introduced, are therefore pressed slightly together and then spring apart from one another as soon as the retaining element has been introduced into the control valve housing far enough to enable the spring arms to snap into the region of larger diameter formed by the undercuts. It is then no longer possible to pull the retaining element out of the control valve housing because the spring arms are supported against the undercuts. The thus described retaining element with its spring arms, which latch in the control valve housing, may advantageously also be used independently of the recesses according to the invention. Suitable undercuts in the control valve housing then have to be provided in some other manner.
The present invention also provides a method of manufacturing a brake booster according to the invention, whereby the previously described recesses in the control valve housing are advantageously produced in that a plurality of first slide bars, which are spaced apart in peripheral direction, are introduced into an injection mould for the control valve housing from the end of the control valve housing, which is to be disposed in the booster housing, parallel to the longitudinal axis of the control valve housing in such a way that said first slide bars in the region of the control valve housing, which is later overlapped by the collar of the plate, take the place of the surface of the control valve housing. In other words, the first slide bars fill the injection mould for the control valve housing at some points of the portion of the control valve housing which will later be overlapped by the collar of the plate, so that no material may flow up to said points. The control valve housing is then injection moulded and the first slide bars, after adequate cooling of the injected material, are withdrawn from the injection mould. The openings required for radial caulking of the collar may be produced in said manner extremely easily and without transversely displaceable elements taking up space. In the method according to the invention, all of the first slide bars are preferably manipulated, i.e. introduced and withdrawn, jointly, e.g. by fastening all of the first slide bars to a common holder.
According to a preferred development of the manufacturing method according to the invention, before the control valve housing is injection-moulded, in the opposite direction to each first slide bar a second slide bar, which is disposed radially outside of the first slide bar, is slid along the associated first slide bar and introduced into the injection mould to such an extent that between its free end and the end of the control valve housing to be disposed in the booster housing a defined gap remains. During the subsequent injection moulding said defined gap is filled by the injected material, with the result that an undercut is produced in the recesses, which are formed by the first slide bars and enlarged by the second slide bars. Said undercut also may therefore be produced without transversely displaceable elements on the moulding tool and hence in an extremely space-saving manner.
In the method according to the invention, the first and/or the second slide bars may be flat or curved in the shape of a cylinder segment. When the slide bars are flat, their thickness and radial positioning dictate how far the openings, which arise in the lateral surface of the control valve housing, extend in peripheral direction. When the slide bars are curved in the shape of a cylinder segment, their curvature corresponds to that of the control valve housing so that the thickness of the slide bars has no influence upon the extension of the resulting openings in peripheral direction.
Since the radial extension of the undercuts for the spring arms of the said retaining element does not have to be great, the second slide bars, the thickness of which determines the radial extension of the undercuts, are preferably thinner than the first slide bars.
There now follows a detailed description of a preferred embodiment of a brake booster according to the invention with reference to the accompanying, diagrammatic drawings. Said drawings show: